Recently there has been increased interest in utilizing cover crops in our corn and soybean production systems because of government sponsored cost share programs and soil health benefits. Concurrently, there has also been increased utilization of soil residual herbicides in our corn and soybean production systems to help manage herbicide resistant weeds such as marestail, pigweeds, and ragweeds. Soil residual herbicides can remain active in the soil for anywhere from weeks to months after application. The length of time a residual herbicide remains biologically active in the soil is influenced by soil type, soil pH, organic matter, rainfall, and temperature. Since these factors will vary from field to field, definitive time intervals of residual herbicide activity can be difficult to predict. A significant challenge has arisen because use of residual herbicides in our corn and soybean production systems may in- terfere with establishment of fall seeded cover crops. An unfortunate coincidence is that many of the crops being used for cover crops were not evaluated for herbicide carryover when field research was being conducted for support of the EPA la- bel of the respective herbicide. As a result, data are lacking regarding rotational intervals for establishment of many cover crop species. Over the last couple of growing seasons we have established experiments designed to evaluate the impact of commonly used residual herbicides on the establishment of many cover crop species. In addition, our colleagues in adjacent states have conducted similar research and we feel like we have a better handle on this topic now than we did two or three years ago. As was mentioned above, predicting herbicide persistence is complicated because so many different factors can influence herbicide dis- sipation in the soil. As a general rule, residual herbicides that have activity on grass weeds can interfere with the establishment of some grass cover crop species, especially the smaller seeded ryegrass species. Residual herbicides from the group 2 (ALS), group 5 (triazine), group 14 (PPO), or group 27 (bleacher) can inter- fere with the establishment of some of the broad leaf cover crop species. Residual Herbicides and Fall Cover Crop Establishment Find the latest weed Management information and tools from Purdue: https://ag.purdue.edu/btny/weedscience Bill Johnson Professor of Weed Science Travis Legleiter Weed Science Program Specialist August 24, 2015